The present invention relates to a contactless IC card which does not have a power supply, battery, or sources of signals such as a clock and a carrier. More particularly, the present invention relates to a contactless IC card which is characterized by a transmit/receive circuit adapted to recover a clock for operating signal processing circuits such as logical circuits, memory, and so on, data, and electric power from a high frequency signal derived from an antenna or a coil arranged on the IC card, and to transmit a portion of data stored in the memory in accordance with the received data through the antenna or the coil as transmission data.
In “A Low-Power CMOS Integrated Circuit for Field-Powered Radio Frequency Identification Tags” by D. Friedman et al. published in 1997 IEEE ISSCC (International Solid-State Circuit Conference) p.294 and 295, SESSION 17/TD: LOW-POWER/LOW-VOLTAGE CIRCUITS/PAPER SA17.5, a RFID (Radio Frequency Identification) tag using a high frequency signal is described. This RFID tag is a passive tag which operates as follows. Upon reception, the tag recovers data and electric power from a 100% AM-modulated RF (Radio Frequency) signal from an antenna through a circuit composed of a Schottky diode (or a Schottky barrier diode) and a capacitor. Upon transmission, the tag changes the impedance of a circuit including the Schottky diode, the capacitor and the antenna to transmit data. Such a passive tag is advantageously manufactured at a low price and in a compact configuration.
FIG. 4 illustrates the configuration of a transmit/receive circuit for recovering electric power and data in the prior art.
(Data Reception and Power Recovery)
A first diode 71 connected between an antenna terminal 70 and the ground and having its anode terminal grounded, has its cathode terminal connected to anode terminals of second and third diodes 73, 72 in parallel. Data and electric power are recovered from an output terminal 76 and a power terminal 77, respectively, through capacitors 75, 74 arranged between cathode terminals of the associated diodes 73, 72 and the ground.
(Data Transmission)
Data is transmitted by opening or closing a switching element 78 arranged at the cathode terminal of the third diode 72 to change a load impedance of an antenna.
In the transmit/receive circuit of FIG. 4, the diodes 71, 72, 73 are mainly responsible for detection and rectification, and the capacitors 74, 75 for accumulation of charge.
The conventional transmit/receive circuit has the second diode 73 for power recovery and the third diode 72 for data recovery connected in parallel to the cathode of the first diode 71, so that a voltage detected and rectified by the first diode 71 is supplied commonly to the output terminal 76 and the power terminal 77. With this configuration, a supply voltage outputted from the power terminal 77 is likely to exceed a data (clock) signal voltage outputted from the output terminal 76, depending on the magnitude of a load connected to the power terminal 77. In such a case, a logical circuit, which receives the supply voltage and data signal from the transmit/receive circuit, is highly susceptible to destruction.
In addition, when the switching element 78 is operated for data transmission, the power diode 73 is largely affected by a change in impedance, which would result in failing to recover a stable supply voltage.